The need to generate high accuracy oscillating signals is necessary in radar, radio and other applications. One of the high accuracy signal generation techniques involves the excitation of crystals which then generated a signal having a frequency characteristic of that crystal. However, this technique is limited to the use of one crystal or, if several frequencies were desired, several crystal circuits for each frequency desired would be necessary. This entails a frequency generating circuit for each crystal and, perhaps, could not be accomplished for certain frequencies because, there are no crystals providing that characteristic frequency.
One technique for solving this problem is frequency synthesis. One technique of frequency synthesis uses frequency multiplication and division circuits in combination to generate a multiplicity of frequencies. In more sophisticated techniques a single oscillating output may provide any of a number of output signals on a single output terminal. One such technique is shown in Stankey, U.S. Pat. No. 4,584,539, issued Apr. 22, 1986. In Stankey, a voltage controlled oscillator is fed a DC signal which causes an output signal having an initial imprecise frequency. The output signal of the voltage controlled oscillator is tapped and fed to a mixer where it is mixed with a comb signal generated by a comb generator feeding from a crystal oscillator output. The closest comb signal to the output of the voltage controlled oscillator will produce a relatively low frequency product. The other combinations are filtered out by a low pass filter. The output of the crystal oscillator is divided down to a frequency which provides the desired frequency output signal when added to or subtracted from one of the singlas provided by the comb generator. The rate of frequency division of the frequency divider is selected by logic circuitry. The output of the frequency divider is provided to a phase detector. The other input to the phase detector is the output of the low pass filter. The output of the phase detector is filtered using a common loop filter. The output of the loop filter is combined with the original input voltage to the voltage controlled oscillator. This phase locked loop stabilizes when the output of the voltage controlled oscillator equals the output required from the frequency synthesizer.
Stankey provides a high accuracy, high agility frequency synthesizer (agility is the ability to switch quickly from one frequency to another over a wide range). However, even when locked, minute traces of other frequency components may be provided which causes excessive residual phase noise in the output frequency. It is an object of the present invention to provide a technique for filtering out this residual phase noise.